1. Field of the Invention
The invention relates to a method and an apparatus for pneumatically conveying bulk material which does not flow readily.
2. Brief Description of the Prior Art
A method of the type mentioned at the outset and an apparatus for carrying it out are known from EP 0 692 411 A1. This known technology uses a method of pneumatically conveying bulk material, in which the latter is transported in a pipe from a departure point by means of a gas stream to a reception point with the bulk material partly filling the pipe in a dense stream having a defined velocity by production of a plug in the pipe and, after closure of the inlet opening of the pipe by means of an inlet closure valve, the pipe being entirely or partly emptied by means of overpressure and with a pseudocontinuous mass flow from the departure point to the reception point being able to be established by continuous repetition of this cycle. The “pump flow method”, in which the pipe is partly filled in suction dense stream operation by provision of a device to generate a partial vacuum and is emptied by switching over to pressure operation and rotary air locks or pressure vessels can thus be dispensed with, is disclosed in EP 0 692 441 B1. Containers and Big Bags can be emptied pseudocontinuously in double action mode when two suction lines are installed in parallel.
A disadvantage here is that in the case of “dead” materials which do not flow readily, the vacuum is not sufficient to fill the suction lines. This causes problems with the closure devices and the transport process.
As an alternative, high-pressure rotary air locks and single/double pressure vessels have hitherto been used according to the prior art or combined with one another as described in DE 199 60 221 C2.
The screw pumps used for some decades are provided with a high power drive motor to convey the plug of bulk material before the closure flap between screw shaft and transport line, which is kept closed by the transport pressure. Screw pumps are mainly used for mineral materials.
DD 267850 A1 shows that conveying screws without a closure flap can also be utilized for feeding bulk material if an input vessel is installed above the screw intake. The screw does not serve to overcome an input pressure difference, which would be associated with leakage of air, but has the task of regulating the loading i.e. the ratio of bulk material to transport air, or keeping it constant.
DE 40 14 912 A1 describes a technique in which the transport tube wall is to be kept free of deposits on the wall by means of a screw-shaped prefabricated partially flexible element having the appropriate direction of rotation so as to promote transport. Here too, the known, customary pressure vessel technology is proposed for the actual filling of the pipe.
It is therefore an object of the invention to provide a method and an apparatus for metering a predetermined amount of bulk material which does not flow readily, emptying it out completely and conveying it to a target point.